Microcystins are peptide hepatotoxins produced by cyanobacteria that commonly form a scum in water bodies including potable water supplies, constituting an environmental public health hazard. Our ultimate goal is to fully explain the mechanism of toxicity of this group of compounds. Microcystins are absolutely liver specific, lethal doses (50-200 migiogram/kg ip) causing loss of the structure of the organ and massive hepatic hemorrhage, followed by death within a few hours. Subacute doses were shown to be very potent tumor promoters causing increased incidence of malignant changes in the liver of rats initiated with diethylnitrosamine. Liver cells are the target of these toxins because these cells, unlike others, have the capacity to take up these peptides. It is proposed that transport therefore is the determinant of the exclusive organ selective toxicity of microcystin. In order to reach a full understanding of microcystin hepatotoxicity the nature. the identity(ies), properties and the control of activity of the transport process need to be addressed which is the focus of this application. The specific aims are: Aim 1. To characterize microcystin transport in rat liver sinusoidal membrane vesicles. These will be used to probe the transport mechanism in isolation from toxic cellular interactions. These experiments will define how the transporter functions in the sinusoidal pole of the hepatocyte and possibly suggest strategies for altering its altering its activity and therefore exposure to the toxins. Aim 2. To use the Xenopus laevis oocyte expression system to characterize and clone the rat sinusoidal microcystin transporter. The nature and structure of the transporter will be determined by expressing it in Xenopus laevis oocytes by injection with rat liver RNA, identifying a the single species of mRNA that is the message and sequencing it. It will be possible to better define the mechanism of the transport at the molecular level and determine what other xenobiotics or metabolites are substrates for the carrier.